1. Field of the Invention
The invention relates to an ultrasonic echography apparatus, comprising a piezoelectric transducer which is composed of M elementary transducers which operate at the central frequency f.sub.c and are connected to an electronic scanning device, a stage for transmitting an ultrasonic beam, and a stage for receiving and processing echographic signals returned to each elementary transducer, said receiving and processing stage comprising a device for forming M channels which comprises, for each channel, an analog-to-digital converter which operates at the sampling frequency f.sub.e.
The invention can be used particularly attractively in the field of medical echography, notably for the formation of images of organs.
2. Description of the Prior Art
The receiving and processing stage of echography apparatus of the kind set forth serves to realize the function which is known as the formation of channels. This operation consists in the focusing of the ultrasonic beam during reception; practically speaking, this is the summing in phase of the M echos received by the M elementary transducers and originating from the same scanning point. Because the various elementary transducers are not situated at the same distance from the focal point, the channel-forming device essentially comprises delay lines which enable compensation for these differences in distance and also enable all echos to be brought in phase for summing.
Echographic scanning takes place in several directions, all directions extending in a plane which extends through the M elementary transducers which are generally arranged in a row in the form of an array which may be either a linear array or a phase array. In the former case, the various scanning directions extend parallel to one another in the scanning plane, and in the case of the phase array these directions imply sectorial scanning. In the two cases, scanning takes place by focusing on several points, proceeding from the nearest to the furthest point for a given direction, followed by the step-wise change-over to a next, adjacent direction. In order to avoid unnecessary complexity of the description, it will limited to the focusing on several points of a given scanning direction. It is also to be noted that in the two cases (linear array and phase array), the receiving stage operates with one channel for each elementary transducer, so M channels for M transducers.
These channels are generally formed in a conventional way by analog processing, each channel being formed by a group of delay lines which can be switched in order to obtain a correct phase for each focal point of a given scanning direction. The signals delayed by different delays are subsequently summed. Some focal points can thus be obtained for each direction, say from 15 to 20 points. With each of these points there is associated a small sharp field (the equivalent of a field depth in optics), enabling the reconstruction of the signal, subsequent to the channel-forming device for the formation of images of organs. The precision obtained using this analog approach, however, may be considered to be insufficient and when a larger number of points is desired per scanning direction, it is very advantageous to utilize a digital channelforming device in which each channel comprises an analog/digital converter which is connected to its input and whose sampling frequency is referred to as f.sub.e.
The frequency f.sub.e must satisfy the Nyquist criterion and, moreover, in order to obtain a high focusing precision, it is advantageous when the sampling interval (the period) is as small as possible, necessitating the possibility of operation at a very high sampling frequency f.sub.e. However, the cost of fast components is such that it is not feasible to increase the frequency f.sub.e very substantially, the more so because the number of components used in analog channel-forming architectures is already very large. A first approach in realising a digital channel-forming device consists in the direct transposition of the analog device by providing each channel with a variable digital delay device such as a shift register or a FIFO memory which is controlled by a programmed memory in order to obtain delays in steps which are variable from one channel to another at a given instant and which are also variable within a given channel) from one instant to another.
The value of these delays whose elementary step is equal to the sampling period 1/f.sub.e will be described hereinafter.
When the desired elementary delay step is smaller than 1/f.sub.e, there is a technical problem in that the output signal of each channel must be enhanced. This technical problem is solved, for example as described in the publication: Journal of the Acoustical Society of America, Vol. 63, No. 2, February 1978, New York, USA, pp. 425-434. The structure of the channel-forming device described in the cited publication remains substantially the same as that of the transposed analog structure, i.e. there is only one digital delay device for each channel. However, in one particular structure an interpolation filter is inserted in each channel to enable enhancement of the output signal of each channel, be it at the expense of a change of frequency between the input and the output of the channel-forming device.